Ultraviolet (UV) radiation can be used to induce an antigen-specific suppression of the immune response. After a single exposure to UV radiation mice are incapable of responding to contact allergens applied epicutaneously to unirradiated skin or to sheep erythrocytes injected subcutaneously. The hypothesis to be tested during the course of this project is: can a single exposure of mice to UV radiation, prior to antigenic sensitization with allogeneic spleen cells, result in the development of an antigen-specific suppression of the immune response to the allogeneic spleen cells. This hypothesis will be tested by sensitizing UV-irradiated mice with allogeneic spleen cells and measuring the effect UV radiation has on the generation of delayed hypersensitivity (DTH) in vivo, and the proliferative resonse of spleen cells from the UV-irradiated mice in a one way mixed lymphocyte reaction (MLR) in vitro. Results from preliminary experiments have indicated that allogeneic sensitizaiton of a UV-irradiated mouse does cause a suppression of the repsonse to alloantigens. Both DTH and MLR are suppressed. The suppression is specific for the antigen used to sensitize the UV-irradiated mouse and suppressor cells are found in the spleens of the UV-irradiatied animals. The specific aims of this project are: 1. To establish an experimental model system to study the suppression of the response to allogeneic antigens. 2. To determine if the survival of allografts can be prolonged by a single exposure to UV radiation. 3. To establish the mechanism by which UV radiation causes suppression of the immune response. The long term goal of this project is to detemrine if a single treatment with UV radiation can be used to induce an antigen-specific suppression of allograft rejection. The experiments outlined in this proposal are designed to determine the feasibility of the long term goal.